Some 87 million Americans and over 290 million individuals worldwide suffer from some form of chronic pain. For centuries opioid drugs have been used for the treatment of pain. However, there are a number of major limitations to the long-term use of opioids including: the development of physiological tolerance, a profound decrease in analgesic effect observed in all patients during prolonged administration; debilitating side-effects; and drug abuse liability potential. Recent evidence from many laboratories has shown that opioid receptors form both oligomers (the mu opioid peptide receptor (MOP-R) interacts with MOP-R and forms a oligomeric complex) and heterodimers (the MOP-R interacts with a different opioid receptor such as the delta opioid peptide receptor (DOP- R). These opioid receptor complexes produce unique ligand binding sites that have different pharmacological profiles compared to the individual receptors alone. The Gallo Center was the first to show that the compound 6'-GNTI interacts specifically at the DOP-R and kappa opioid peptide receptor (KOP-R) heterodimeric complex (Waldhoer et al., 2005). This heterodimeric specific ligand caused analgesia when delivered into the spinal cord but not when it was delivered into the brain, thus representing a novel therapeutic strategy to limit the degree of side effects and drug abuse liability potential. Specifically targeting opioid receptor heterodimeric binding sites changes the way we view traditional pharmacology and offers new tools for drug discovery that may potentially lead to the generation of novel compounds. The goal of this proposal is to develop a drug discovery platform that targets binding sites in opioid receptor heterodimeric complexes but not individual opioid receptors. We will specifically develop a high-throughput calcium mobilization assay that targets the MOP-DOP, MOP-KOP or DOP-KOP receptor heterodimer complexes using validated and characterized stable opioid receptor heterodimeric cell lines. The overall aim will be to develop opioid receptor heterodimer-specific drugs that have greater tissue specificity, limited tolerance and dependence and improved side effect profile with reduced drug abuse liability. The goal of this proposal is to identify novel pain therapeutics by developing a drug discovery platform that targets binding sites in opioid receptor heterodimeric complexes but not individual opioid receptors. The aim will be to find novel compounds for the treatment of pain that have increased tissue specificity with limited side effects and drug abuse liability potential. [unreadable] [unreadable] [unreadable]